issue with prorating variable pay

Hi,

In the company we have a variable pay program in which the payment of the "bonus" is made based on the rating that comes from the performance module, however there are cases in which some employees are only part of the program - plan by half of the duration of the program, then the calculation of the bonus is made based on the salary of the employee at the date he / she stopped being part of the program, without apportioning for the number of days. Then I would like to know if it is possible that I can perform the calculation of the bonus in variable remuneration, avoiding the apportionment by assignment.